Post-partum depression causes maternal suffering and can have a lasting negative influence on the child's cognitive and emotional development. Interventions that prevent or reduce the incidence of post-partum depression would be of significant value to society. Preliminary studies indicate that dietary calcium supplementation during pregnancy may provide an inexpensive, safe, and effective protection against postpartum depression while at the same time providing a number of other health benefits for the mother and child. We will conduct a rigorous test of the hypothesis that supplemental calcium during pregnancy reduces the incidence of post-partum depression in a randomized, double-blind, placebo-controlled clinical trial of a 2 g/day oral calcium supplement in 238 women at risk for postpartum depression as a consequence of a positive history of depression. Supplementation will begin between weeks 16 and 20 of gestation and continue through 12-weeks post partum. Women will be recruited by newspaper advertisements and clinic referrals. Prior to randomization, a computerized version of the SCID followed by a psychiatric diagnostic interview will be used to determine baseline clinical status. Those currently suffering from major depression, active substance abuse, active psychosis, schizophrenia, bipolar disorder or are on antidepressant medication, will be referred for treatment and will not be included in the study. Prior history of postpartum depression and parity will be controlled for during randomization. The antenatal version of the Edinburgh Postnatal Depression Scale (EPDS) will be used to screen for depression at 26, 32 and 38 weeks of gestation and the postnatal version will be used at 6 and 12 weeks postpartum. Those with a positive change score of 30 percent or a score greater than 16 on the EPDS at any screening visit will be evaluated using a psychiatric interview. Those suffering from major depression will be referred for treatment. The proportion of women in the calcium supplemented group who experience a first episode of major depression since entry into the study in the postpartum period, either at 6 or 12 weeks, will be compared to the proportion in the placebo condition. A significantly smaller proportion in the calcium group will be taken as evidence in support of the hypothesis. Positive results from this trial will provide a strong foundation for a multicenter intervention trial.